The broad and long-term objectives of this research are to provide a molecular level understanding of the family of adenosylcobalamin (Coenzyme B 12)-dependent and related enzymes that catalyze C-H bond activations, followed by rearrangement or reduction reactions, in nature. This family of enzymes includes those with B 12, Fe2, Mn2 (and probably other, yet to be discovered) active sites. Recent evidence suggests that these enzymes use an unusual, and still poorly understood, radical chain mechanism of action; the wide distribution of these enzymes in bacteria, plants, and mammals including humans attests to the importance of this radical chain mechanism. The Coenzyme B12 cofactor is essential for the normal maturation of erythrocytes; in man, insufficient B12 leads to pernicious anemia and clinical features that include megaloblastic anemia, malignant neoplasms, and neurological disorders. More specifically, adenosylcobalamin is an essential cofactor for methylmalonyl-CoA mutase and methylcobalamin is an essential cofactor for homocysteine methyltransferase. The specific aims outlined are designed to provide a chemical paradigm for the individual, elementary mechanistic steps thought to operate in the Coenzyme B12-initiated radical chain reaction. Furthermore, one of the steps, the initial homolysis of Coenzyme B12's Co-C bond, is subject to an unusually large, 1012, enzymic rate enhancement. Understanding this enormous effect, in this perhaps prototype system, is another focus of these studies. Lastly, studies are outlined that involve a central Coenzyme B12 With one or two covalently linked polypeptide a-helices. This work, which takes advantage of the nearly ideal features of the B12 cofactor, begins to probe the very important and general but ill-understood question of how polypeptide and protein low frequency motions couple to an enzyme catalysis reaction coordinate, such as the simple (and thus nearly ideal) Co---C cleavage reaction coordinate. Such protein-assisted dynamics are essential components of the enzymic transformations that sustain living organisms, yet remain poorly understood.